Recently, a new technology has emerged known as spintronics. Instead of conventional charge-based electronic devices, this technology uses electron spin to carry information, and thus offers opportunities for a new generation of spin devices. These devices will have the potential advantages of non-volatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices.
The giant magnetoresistance effect (GMR) was discovered in 1988, and is considered to be a very promising technology for spintronics devices. GMR is a quantum mechanical effect observed in layered magnetic thin-film structures composed of alternating layers of ferromagnetic and nonmagnetic layers. When the magnetizations of the ferromagnetic layers are parallel, the spin dependant scattering of carriers is minimized and the material has its lowest resistance; in contrast, when the magnetizations are anti-parallel, the spin dependant scattering of carriers is maximized and the material displays its highest resistance. The simplest GMR structure is the spin valve, which consists of two ferromagnetic layers sandwiching a thin nonmagnetic conductive layer. The flow of electrons in the spin valve is controlled by changing the direction of the magnetization of a part of the device. In previous spin valve structures, one of the two ferromagnetic layers was “pinned” by placing an anti-ferromagnetic layer in intimate contact with the pinned layer. The other ferromagnetic layer was “free” layer whose magnetization could be changed by applying an external magnetic field.
The basic principle GMR effect may be explained using a simple quantum mechanics picture, as illustrated in FIG. 1. In the ferromagnetic layers 10, 11A, the electron energy bands of spin up and spin down are split, which results in an unbalanced density of states for spin up and down at Fermi level EF. In the middle nonmagnetic layer 12, the density of states for spin up and spin down are even. The conductivity of the GMR structure may be expressed as follows:σ∝n1↑n2n2↑+n1↓n2n2↓  (1)where n is the density of states of spin up or down in the nonmagnetic layer 12 at the Fermi level, n1↑, n1↓ is the density of states of spin up, spin down respectively in the left ferromagnetic electrode (10), and n2↑, n2↓ is that corresponding to the right ferromagnetic electrode (11A). In FIG. 1A, the magnetizations of the two electrodes (the two ferromagnetic layers 10, 11A) are all in the up direction (parallel); the spin up electrons are the majority carriers, and the spin down electrons are the minority carriers. The magnetization of electrode 11A is reversed to the state shown as electrode 11B in FIG. 1B, however, and the spin up, down electrons become the minority, majority carriers, respectively. Assuming n+, n− as the density of states of for the majority, minority cases, respectively, the conductivity of parallel and anti-parallel structures may be written as:σp∝n2(n1+n2++n1−n2−)σap∝n2(n1+n2−+n1−n2+)  (2)
The spin polarization is defined as P=(n+−n−)/(n++n−), and giant magnetoresistance is defined as GMR=(σp−σap)/σap. From equation (2), one may derive:
                    GMR        =                              2            ⁢                          P              1                        ⁢                          P              2                                            1            -                                          P                1                            ⁢                              P                2                                                                        (        3        )            This is same as the Julliere formula for tunneling magneto-resistance.